debug-on-entry
A second way to start debug on a function is to enter the
debugger when you call the function. You can do this by calling
debug-on-entry.
Type:
M-x debug-on-entry RET triangle-bugged RET
Now, evaluate the following:
(triangle-bugged 5)
Emacs will create a `*Backtrace*' buffer and tell you that it is
beginning to evaluate the triangle-bugged function:
---------- Buffer: *Backtrace* ---------- Entering: * triangle-bugged(5) eval((triangle-bugged 5)) eval-last-sexp(nil) * call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
In the `*Backtrace*' buffer, type d. Emacs will evaluate
the first expression in triangle-bugged; the buffer will look
like this:
---------- Buffer: *Backtrace* ----------
Beginning evaluation of function call form:
* (let ((total 0)) (while (> number 0) (setq total ...)
(setq number ...)) total))
triangle-bugged(5)
* eval((triangle-bugged 5))
eval-last-sexp(nil)
* call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------
Now, type d again, eight times, slowly. Each time you type d, Emacs will evaluate another expression in the function definition. Eventually, the buffer will look like this:
---------- Buffer: *Backtrace* ----------
Beginning evaluation of function call form:
* (setq number (1= number)))
* (while (> number 0) (setq total (+ total number))
(setq number (1= number))))
* (let ((total 0)) (while (> number 0)
(setq total ...) (setq number ...)) total))
triangle-bugged(5)
* eval((triangle-bugged 5))
eval-last-sexp(nil)
* call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------
Finally, after you type d two more times, Emacs will reach the error, and the top two lines of the `*Backtrace*' buffer will look like this:
---------- Buffer: *Backtrace* ---------- Signalling: (void-function 1=) * (1= number)) ... ---------- Buffer: *Backtrace* ----------
By typing d, you were able to step through the function.
You can quit a `*Backtrace*' buffer by typing q; this quits
the trace, but does not cancel debug-on-entry.
To cancel the effect of debug-on-entry, call
cancel-debug-on-entry and the name of the function, like this:
M-x cancel-debug-on-entry RET triangle-debugged RET
(If you are reading this in Info, cancel debug-on-entry now.)
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